Insulin-like growth factors (IGF's) have been identified in various animal species as peptides that are biologically active in growth, e.g. via proliferation of cells. They are believed to mediate effects of somatotropins and possibly other hormones. The designation "insulin-like growth factor" was chosen to express the insulin-like structures and effects of these peptides. IGF's have nearly 50% homology with insulin. In three dimensional structure they resemble proinsulin, i.e., they are single-chain peptides cross-linked by three disulfide bridges and containing an A-chain portion (A domain), a B-chain amino-terminal portion (B domain) and an A-B connecting chain (C domain). A carboxy-terminal extension (D domain) not found in proinsulin is also present in at least some IGF's.
Several classes of IGF's have been identified in animals. Normally these include IGF-I, IGF-II and others. Circulating levels of these peptides appear to be under the control of somatotropin to some extent, with IGF-I controlled to a greater extent than IGF-II. In various cell culture systems, IGF's have shown mitogenic effects measured, e.g., by increased tritiated thymidine incorporation.
It has been demonstrated that in some animals, at least two sets of IGF receptors exist, one preferentially binding IGF-I and the second IGF-II, suggesting separate functions for IGF-I and IGF-II. However, the biological functions of IGF-II appear to vary among mammalian species. For example, while rat IGF-II levels have been found 20-100 fold higher in fetal than maternal circulation, human serum IGF-II in the fetus is normally lower than in adults.
Because of its potential bioactivity and utility for enhancing desirable cell growth in animals, the amino acid sequence of bovine IGF-II ("bIGF-II") has long been sought together with a more detailed understanding of its growth-promoting and other activities, its active fragments, etc. Heretofore, neither that sequence nor the DNA sequence of the bIGF-II gene has been reported.
Studies with rat and human genomic libraries suggest that IGF-II genes contain at least four exons. The large size and complexity of the genes for human and rat IGF-II have made their isolation and identification so difficult that the DNA sequences of those genes have not yet been fully determined. For purposes of making and studying bIGF-II, however, there has been a need to isolate and determine the complete DNA sequence of the bIGF-II gene.
Accordingly, it is an object of this invention to provide highly purified and/or synthetic peptides having one or more of the biological activities of bIGF-II and, more generally, such peptides consisting essentially of amino acids providing such activity, or peptides which can be readily converted to those having such activity.
It is another object of this invention to provide methods using such peptides to promote desirable growth or functionality of cells in animals including, e.g., muscle and/or mammary epithelial cells.
Another object of this invention is to provide DNA useful in making such peptides.
Another object of this invention is to provide processes utilizing such DNA in the production of such peptides.
Other objects will be apparent from the detailed description herein and the appended claims.